The first four months of the state fiscal year usually produce 32 percent to 33 percent of the annual sales tax haul, noted Eva DeLuna Castro, budget analyst for the center-left think tank the Center for Public Policy Priorities.

Fiscal 2017 began Sept. 1.

At current collection rates, the state would wind up the year with between $1.3 billion to $2 billion less than the $30.5 billion of sales tax that Hegar predicted in October 2015. At the time, the Republican comptroller was certifying there'd be enough revenue to cover the two-year budget that the Legislature enacted in 2015.

"It is good," DeLuna Castro said of December's $2.44 billion in sales tax collections. "But it needs to get a lot better in order not to end the biennium with a serious sales-tax shortfall."

Texas' 6 ¼-cent sales tax generates 58 percent of state tax revenue.

Hegar noted that last month, there was a healthy increase of 7.5 percent in collection of motor-fuels taxes, compared with December 2015.